1. Field of the Invention
The present invention relates to systems for protecting electrical circuits, and in particular to an arcing fault detector which detects the presence of an arcing fault within an enclosed electrical equipment unit.
2. Description of the Prior Art
Arcing faults may be sub-classified as line-to-line faults or ground faults. There are several methods in present use by which these faults are detected and cleared. For ground faults the use of ground fault detectors in combination with window type current transformers is generally considered to be the most efficient and reliable method. The principle used in this method consists in monitoring the currents in the supply and return conductors of the circuit through a window type current transformer, the sum of the currents normally being equal to zero. The presence of a net current when all supply and return conductors are added together would indicate that some of the current is flowing through ground, which is a telltale of a ground fault. In this case the supply circuit breaker(s) is tripped open. The settings of the associated ground fault detectors can be very low (on the order of 3 percent of circuit capacity) because normally there is no intentional current flow through ground. Other methods of ground fault detection are less critical than the above. The use of an inverse-time-overcurrent relay in the neutral conductor of a grounded system cannot be set to detect low currents because of the normal neutral flow that occurs with phase unbalanced loads.
The detection of single phase or three phase line-to-line faults is accomplished by use of inverse-time-overcurrent relays or current differential relaying systems. The overcurrent relays are sensitive only to fault currents which exceed the circuit rated capacity by predetermined factors, following an inverse time characteristic. The higher the fault-to-rated-current ratio, the faster the clearing time would be. In circuits having more than one circuit breaker in series the sensitivity and speed of operation of the overcurrent relays have to be coordinated to permit the downstream protective devices to trip first. This results in a reduction in sensitivity to the supply circuit breakers.
The overcurrent relays are fully satisfactory in detecting faults having high current flow. The localized fault impedance is very low, and the current is limited by the distributed circuit impedance or the power source impedance. Fault induced heat is similarly distributed and not localized in the faulted enclosure.
The current differential relaying system operates on the principle of summation of all currents flowing in and out of its zone of protection and acts instantaneously to clear any fault. The sensitivity of this system is only limited by the matching of the saturation characteristics of the associated current transformers. In order to avoid nuisance tripping of circuit breakers under faults out of the zone of protection, but where fault current flowing through the circuit in question causes saturation of current transformers, the transformers must be perfectly matched or the sensing level must be increased to prevent relay pick-up caused by mismatch. The percentage differential relaying system improves on this condition. However, the sensitivity of relaying systems of this type is still limited to more than 5 percent of circuit rated capacity.
In arcing faults the resistance of the fault path limits the current to values that are often lower than the rated circuit capacity. If the circuit is not fully loaded with normal load when the fault is incepted, fault current may not even reach the rated full current value. The heat dissipated in the concentrated fault resistance is expressed in the formula: EQU H=I.sup.2 Rt
where I is fault current, R is fault resistance, and t is time in seconds. As a example, a line-to-line fault adding up to 100 amperes in a three phase 480 volt system would generate heat at a rate of 48 KW. If the protective device would take 10 seconds to clear, the internal heat of the faulted enclosure would be raised by 480,000 joules or 455 BTU.
Experience has shown that these faults tend to remain unchanged at low current values for relatively long periods. The concentrated dissipation of heat for long periods is what makes arcing faults so destructive to electrical equipment which relies on overcurrent relays for protection. The superior protective capabilities of the current differential relaying system are not regularly used because of the relatively high cost and complexity of such a system. In addition, a fault having the magnitude of the above example (100 amperes) incepted in a high current circuit (say 4000 amperes rated) would fall below the practical sensitivity of present state-of-the-art current differential systems. However, a fault of that magnitude is quite capable of destroying the average low voltage electrical equipment unit.